There has been known an image forming apparatus including a photosensitive drum, on which an electrostatic latent image is formed and a developing roller which contacts the photosensitive drum to supply developer to the photosensitive drum (for example, refer to JP2007-93950A). In this technique, generally, a direct current voltage is applied to the developing roller, so that developer is moved from the developing roller to the photosensitive drum.
However, according to the above technique, when a temperature of the developing roller is lowered, for example, the developing roller becomes hard, so that a following capability of the developing roller with respect to the photosensitive drum is deteriorated and side end portions of the developing roller may be thus partially separated from the photosensitive drum. In this case, developer carried on side end portions of the developing roller are difficult to move to the separated photosensitive drum, so that a side end portion of a sheet is partially not printed (hereinafter, such problem is referred to as ‘white out in a sheet side end portion’).